576 BACTERIA OF PLANT DISEASES. 



injury, and thus normally get inside their hosts. The distribution from 

 flower to flower and tree to tree is through the agency of insects, mainly 

 flower- visiting source. Infection also occurs on the young shoots and less 

 frequently on the fleshy bark through injuries. Insects and birds are agents 

 of distribution and inoculation in these cases. No evidence could be found 

 that the germs are carried by the wind. The blight germs usually die out in 

 the twigs which are blighted and dead, but in certain cases the germs manage 

 to keep alive through the summer by making slow progress in the fleshy 

 bark. Such cases may succeed in living over winter. Winter weather is 

 favorable to the longevity on account of the moisture and low temperature. 

 The cases of "hold over" blights start oft' again in spring and exude quanti- 

 ties of gummy matter full of the bacilli. This is visited by insects, especially 

 flies and wasps, and carried on to the newly opened flowers, thus completing 

 the life cycle. 



' ' An oval rod-like bacillus 0.6-0.8 n by 1 to 6 // long. Constant in diameter 

 but varying greatly in length. Occurs singly or in young cultures in pairs, 

 chains, or masses. Stains readily with the ordinary aniline dies either watery 

 or alcoholic solutions. Has 110 capsule, but is supplied with several flagella 

 scattered over the surface. It is actively motile. Does not produce spores. 

 On nutrient beef and potato broth produces first a strong turbidity and a 

 slight granular pellicle on the surface, which breaks up and settles to the 

 bottom. The color of the mass is milky white on all solid media. 



"On agar plates the outside colonies at ordinary temperature (18 to 20 C.) 

 reach a diameter of about one millimetre in forty-eight hours, and at the end 

 of a week become five to six millimetres across. A temperature of 36 to 

 37 C. starts the growth more promptly, but results in a feebler ultimate de- 

 velopment. 



"The addition of malic or citric acid in small amounts so as to acidify 

 the agar, increases the vigor of growth, while an excess of alkali diminishes 

 it. On gelatin made from the commercial brands the opposite effect is pro- 

 duced. Gelatin should be neutral to phenolphthalein. to insure vigorous 

 development. There is moderate liquefaction in good gelatin culture. A 

 moderate growth is made on sterile potato cylinders. 



"In the fermentation tube it decomposes sugar without the formation of 

 gas. It is most vigorous on maltose, the cultures becoming strongly acid, 

 and is slightly less so on cane sugar, dextrose, and levulose. It is aerobic and 

 facultative anaerobic. It produces no pigment or coloring matter of any sort, 

 and no odor. It does not decompose starch. Its principal food consists of 

 nitrogenous matter, sugars, and probably, to some extent, certain organic 

 acids, the very substances which occur in vigorous, young, growing tissues 

 of the host. Certain statements formerly made are now known to be erro- 

 neous. 



"The germ mass is said to be yellowish -white on potato. This could 

 only come from an impure culture, as the true pear-blight germ is always 

 white. Gas, in some places COs, is said to be formed. This never oc- 

 curs. Butyric acid is said to be one of the products of its decomposition. 

 The germ produces acid but never butyric. Starch is said to be decomposed 

 and used as a food, but so far we have never been able to demonstrate this. 

 The germ is said to live over winter in the soil. In our search we have 

 failed to find it in such places, and its life cycle is complete without it" (Waite). 



BACILLUS TRACHEIPHILUS (Smith). 



The cause of "wilt " in various species of Cucurbitacece cucum- 

 bers and melons. 



"Bacilli, often two or three times as long as broad, of medium size; soli- 

 tary or in pairs, occasionally in chains of four. The dimensions vary greatly 

 in the infected plant ; many rods are 1.2 to 2.5 // long by 0.5 to 0.7 p broad 



